Originally published at: Astonishingly weird video of commercial airliner freezing in mid air | Boing Boing
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I learned about this in some math class, were it was framed as a handy way to determine whether your sailboat was going to pass in front of or behind another boat (or on course to crash) when the two boats are on intersecting headings.
- If the other boat is moving faster than the distant background, it will pass in front of you
- If the other boat is moving slower than the background (i.e., appears to be moving backwards relative to the background), you will pass in front of it. (Which is of course a similar effect to what we see here.)
- If the other boat is static relative to the background, you’re gonna crash unless one of you does something to prevent it.
Naturally things are a little different if you’re on parallel or divergent headings, but of course you can observe similar parallax effects.
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Yup, as a kayaker I’ve avoided collisions with powerboats using this very principle.
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I think we have all seen birds catch a breeze and hover in the same place without flapping their wings. It is interesting that we rule out that possibility for man made aircraft and go straight to optical illusion.
Even landing with flaps fully down, the airspeed of an airliner is such that the headwind required to cancel its forward groundspeed would be unrealistically enormous, and would also affect the plane from which the video is being taken. Optical illusion is the only explanation that makes sense here.
(If you look carefully, you can see forward motion with respect to the background - it’s just not super-obvious.)
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Your eyes are not playing tricks on you!
It’s actually an optical illusion!
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Learned the same thing as a pilot, a plane that appears to be stationary relative to the ground/clouds is a collision risk.
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Technically true I guess. Your brain is playing tricks on you.
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There is something similar called Constant Bearing, Decreasing Range, or CBDR, when there are two object on converging vectors and from one point of view the other seems to stay at the same angle as they get closer. I was once landing in jet on a runway that was parallel to a small plane runway. I looked out the window to see a Cessna that seemed to be flying backward.
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The third case also applies to cars traveling towards an intersection - if the other car does not appear to move relative to a fixed point in your car (such a roof pillar), then you are on a collision course. Knowing this saved me from what would have been a devastating accident on the driver’s side several years ago when the other car blew through their stop sign at 100 km/h. I had right-of-way and no stop sign.
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I live under the easterly approach path for 27C & 27L at O’Hare, and when driving about, I’ll encounter this from time-to-time, and even though I rationally know what’s going on, the irrational part of me is all “what is this sorcery?!??!?!”
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“Have you tried turning it off and on again?”
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Sorry, the plane really is stopped in midair. It feels like it happens on every flight I take.
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Combine this problem with the absurdly large A-Pillars in modern cars, and it’s no wonder there are an increasing number of car/pedestrian and car/bicycle accidents. I slow down a lot – and I’m happy to – when I cross the bike/ped path near my house, but it’s remarkable how easy it is to hide a person or a bicycle at a very modest distance. (And I have quite practical Honda CR-V, no giant truck).
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Brings to mind surfing at a standstill:
Except that airplanes don’t fly by flapping their wings, so it’s pretty easy to rule out that possibility.
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I fully understand the principle of the optical illusion that some people think is the culprit here. Which is why I really think this is CG. Given the minimum speed that the plane in question has to be moving at, I don’t see how an optical illusion would be enough to account for the extremely uncanny lack of motion. I would expect it to look very slow, but not stopped. Maybe I just underestimate the potency of this particular illusion. I have definitely seen too many “look at this completely non-faked video I just took with my phone!!1!” posts on the internet.
I was taught something similar except using the foreground instead of the background.
Specifically, note the position of the other boat relative to two stanchions. If the other boat crosses the forward stanchion, it’ll pass in front of you. If the other boat crosses the astern stanchion, you’ll cross in front of it. If it remains between the two stanchions, you’ll collide.
It has the benefit of working in open water.
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This is a nice one. The context I learned this one in made it clear that it was meant for some small-body freshwater setting, but I always wondered what you’d do if you didn’t have a background to compare against. Comparing against the vehicle itself makes a lot of sense since the background at infinity is going to be static relative to the vehicle.
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